To chime in on that, Calypso i.e the LDC+Clang equivalent of
what you described isn't dead (it just revived a few weeks ago
from a long slumber) and it should be possible to make a
smaller codegen-less version for DMD that only depends on a
few Clang and LLVM support libraries. Another possibility..

Good ideas! Some things to consider:

1. I'd like this to be pluggable, so that different compilers
can be used without being disruptive to dmd.

2. DMC++ is not only a C compiler, but a C++ compiler. The C++
compiler, though, is stuck on C++98 and is unlikely to see much
improvement on that.

3. BetterC is called BetterC and not BetterC++ because D's C++
interop is more fragile than the C interop. I felt it better to
nail the C end first.

4. It certainly would be nice to be able to insert C++ headers,
too, but I'd expect more problems (!).

5. C is stuck at C99. Being a fixed target makes it more
tractable. Supporting C++ means constant revisions and high
support costs.

C is currently at [C11]. Granted not many projects switched to
it, as its changes were not as useful as C99. All the things
added to C11 are features that try to extend the language in
directions where it's better to switch to a better language like
D (-betterC).

6. Using DMC++ as a front end would add somewhat less than a
megabyte (half?) to the binary size of dmd. I have no idea what
Calypso or other solutions would add. If they are large, they
may only make sense as a separate executable as a filter.